1. Field of the Invention
This invention relates to polyester polyols which are the hydroxyl-terminated reaction products of (a) terephthalic acid residues and (b) polycarboxylic acid-containing polyols. The polyols are useful in the preparation of cellular foam materials, particularly polyisocyanurate and polyurethane foams.
2. Description of the Prior Art
The preparation of foams characterized by isocyanurate and urethane linkages is well known in the art. Generally, these foams are prepared by reacting an organic polyisocyanate with a polyol in the presence of a blowing agent and a catalyst(s). Polyester polyols of many types can be used as the polyol components in the production of these foams.
U.S. Pat. No. 4,039,487, for example, discloses the use of aromatic polyester polyols to prepare polyisocyanurate foams. Although the foams of this patent have good fire resistance and low smoke evolution upon combustion, they have a relatively high friability. Furthermore, the polyols are comparatively expensive to manufacture.
U.S. Pat. No. 4,092,276 also discloses the use of rather costly aromatic polyester polyols in preparing polyisocyanurate foams. Another disadvantage of these foams is that they do not have especially high compressive strength. A further problem with using aromatic polyester polyols, particularly those of low molecular weight, is that the polyols tend to be solid at room temperature, or to be characterized by very high viscosity and poor solubility in resin mixtures, thus making them difficult to handle.
To remedy the above drawbacks, it has been proposed to U.S. Pat. No. 4,237,238 to use in the preparation of polyisocyanurate foams a minor amount of a cheap by-product type of liquid polyol mixture which is obtained by the transesterification, with a glycol of molecular weight from about 60 to 400, of a dimethyl terephthalate esterified oxidate reaction product residue. The polyisocyanurate foams produced are disclosed to be characterized by a high degree of fire resistance with low smoke evolution on combustion, a low foam friability and reasonably good compressive strength.
Further, U.S. Pat. No. 4,346,229 describes how liquid aromatic polyester polyols having excellent shelf stability and compatibility and trichlorofluoromethane can be produced by reacting DMT process residue with an excess of dipropylene glycol. However, deficiencies have been encountered in the fluidity of these polyester polyols, leading to difficulties in employing them in foam production.